A wireless power transfer system includes a power transmission coil device and a power reception coil device and realizes wireless power transmission using electromagnetic induction, magnetic resonance, and the like between coils. As an application example of the wireless power transfer system, there is a power transfer system of an electric vehicle and a plug-in hybrid vehicle. In this case, the power reception coil device is mounted on a vehicle.
In the wireless power transfer system, a power reception coil and a power transmission coil may be arranged vertically with a gap between the power reception coil and the power transmission coil, at the time of wireless power transfer. In this case, because there is the gap between the power transmission coil device and the power reception coil device, a foreign object may enter the gap. Particularly, when a foreign object made of an electrically conductive material such as a coin and an iron nail enters a portion between the power transmission coil device and the power reception coil device, power feeding efficiency may be deteriorated. For this reason, a mechanism for detecting the foreign object entering the portion between the power transmission coil device and the power reception coil device is demanded.
Patent Literature 1 discloses a foreign object detection device in which linear electric wiring lines are arranged in a comb shape to be alternately lined up and it is determined whether there is a foreign object by detecting whether there is a short circuit between the electric wiring lines. Patent Literature 2 discloses a wireless power transfer device (wireless power transfer system) in which a foreign object detection coil is provided between a power transmission coil and a power reception coil and it is determined whether there is a foreign object on the basis of an induced voltage across the foreign object detection coil. The wireless power transfer system of Patent Literature 2 has a configuration of FIG. 32. FIG. 32 is a cross-sectional view along a vertical plane of the wireless power transfer system.
The wireless power transfer system of FIG. 32 includes the power transmission coil 131 and the power reception coil 133, a cover 135, detection loops 137 (detection coils), and a foreign object detection unit 139. Each of the power transmission coil 131 and the power reception coil 133 is formed in a spiral shape in a same plane orthogonal to a plane of paper of FIG. 32. The detection loops 137 are located between the power transmission coil 131 and the power reception coil 133 arranged in a vertical direction as illustrated in FIG. 32. The cover 135 covers the power transmission coil 131 of the lower side and the detection loops 137 from the upper side. The plurality of detection loops 137 are disposed in the plane orthogonal to the plane of paper of FIG. 32. Magnetic flux generated by a current (hereinafter, referred to as a current for power transmission) flowing through the power transmission coil 131 penetrates the detection loops 137. The magnetic flux penetrating the detection loops 137 changes when a foreign object made of an electrically conductive material is placed on a top surface of the cover 135. The foreign object detection unit 139 detects that there is the foreign object on the top surface of the cover 135 by detecting the change. In addition, Patent Literature 3 discloses various shapes of foreign object detection coils.